The present invention relates to an optical deflection device which deflects a laser beam for scanning and to an image forming apparatus which forms images by means of a laser beam by the use of the optical deflection device.
In an image recording apparatus such as a laser beam printer, a laser beam is caused to strike upon a polygon mirror which rotates at high speed, based on information obtained by reading the image as a writing means, and reflected light is projected on the surface of a photoreceptor on a scanning basis for recording images.
FIG. 11 is a sectional view showing the structure of a scanning optical device in the prior art wherein a light beam is deflected for scanning when a polygon mirror is driven to rotate.
When a polygon mirror is rotated at a low speed, it is fixed directly on a rotary shaft of a driving motor to be used. When the polygon mirror is rotated at high speed, however, it is driven to rotate through an air bearing wherein the polygon mirror is fixed on an outer tube of a radial shaft which floats without touching an inner tube of a radial shaft to rotate. The inventors of the invention disclose technologies about an optical deflecting device having therein a dynamic pressure bearing, in TOKKAIHEI Nos. 7-24343, 7-259849, 8-114219 and 8-121471.
FIG. 11 is a diagram showing the sectional structure of optical deflecting unit 100 representing an optical deflecting device having dynamic pressure bearing 110 as a bearing means composed of upper thrust plate 111, lower thrust plate 112 and radial shaft inner tube 113. In FIG. 11, the dynamic pressure bearing 110 is composed of center shaft 114 of casing 101, radial shaft inner tube 113 structured to be solid with the center shaft 114, upper thrust plate 111 and lower thrust plate 112. Coil 115 constituting static magnetic field of a motor is fixed on casing 101 serving as a supporting member. Ring-shaped magnet (permanent magnet) 121 for rotating magnetic field, outer ring section 122 made of aluminum, radial shaft outer ceramic tube 123, rotary polygon mirror 124 and mirror holder 125 are assembled solidly and concentrically as rotor 120 wherein the rotary polygon mirror 124 is sandwiched between the outer ring section 122 and the mirror holder 125. The rotor 120 is fitted to the radial shaft inner tube 113, and the upper thrust plate 111 is fixed on the center shaft 114. When the rotor 120 is rotating, there are formed clearances S of about 3-10 .mu.m between the group of the radial shaft inner tube 113, the lower thrust plate 112 and the upper thrust plate 111 and the group of the upper and lower surfaces and inner circumferential surface for fitting of the radial shaft outer tube 123, thus, the rotor 120 can continue rotating smoothly without touching the dynamic pressure bearing 110 while floating in the air.
Namely, in company of the rotor 120, polygon mirror 124 also rotates, and a laser beam emitted from a laser unit is deflected toward an unillustrated photoreceptor for scanning.
Casing 101 for optical deflecting unit 100 composed of the rotary polygon mirror 124, the dynamic pressure bearing 110 and the rotor 120 is formed to be one body through an aluminum die casting, and an upper opening is covered with cover 102 made of a sheet metal or a synthetic resin plate.
When an air bearing having the structure stated above is used, it is possible to rotate a polygon mirror at a rate of tens of thousands rpm, and as a result, an image forming apparatus such as a high speed digital copying machine or laser printer has been realized.
However, when a polygon mirror is rotated at high speed, heat is generated in large quantities, and thereby the temperature of an optical deflecting device and temperature around the optical deflecting device in an image forming apparatus in which the optical deflecting device is mounted are raised.
When the amount of heat generated from the optical deflecting device is large, deterioration of surface accuracy of a polygon mirror caused by thermal deformation and fluctuation of rotation of the rotor 120 are generated, and thereby uneven scanning and image distortion are caused on outputted images to deteriorate quality of images. This phenomenon is conspicuous especially when enhancing recording density by rotating the polygon mirror 124 at high speed.
When a cooling device is provided on an optical deflecting device additionally as measures for the aforesaid problems, the number of parts in the optical deflecting device is increased, resulting in another problem that assembly man-hour is increased, cost is increased and an optical deflecting device needs to be large in size.
In addition, in the optical apparatus employing a laser scanning optical system such as an image reading device, miniaturization or cost reduction of an apparatus has been pursued. FIG. 12 shows a conventional example of an optical detecting device which makes an optical beam to scan at prescribed angle for scanning in a laser optical unit.
In FIG. 12, polygon mirror J1 is fixed on polygon mirror supporting member J2 by holding member J7. The polygon mirror supporting member J2 is supported by shaft J3 and is supported by bearing J6 through electromagnetic actions of coil J5 and magnet J4 to rotate. The magnet J4 is fixed on vertical supporting arm J21 extended from the polygon mirror supporting member J2.
The optical deflecting device mentioned above has the structure to fix polygon mirror J1 and magnet J4 on the polygon mirror supporting member J2. Since a magnet is fixed on a supporting member which is bent at right angles, therefore, the structure for supporting polygon mirror J1 is complicated, and polygon mirror supporting member J2 and holding member J7 are needed, resulting in a large number of parts, cost increase and difficulty in miniaturization.
Due to an employment of the air bearing stated above, it has become possible to rotate a polygon mirror at a rate of tens of thousands rpm, resulting in realization of a high speed digital copying machine and a laser printer.
In the optical deflecting device having a rotating body which rotates at high speed, it has been found that heat is generated in large quantities with rotation, and thereby the temperature of an optical deflecting device and temperature of the apparatus portion around the optical deflecting device are raised, which is not preferable. When a cooling device is provided on an optical deflecting device additionally as measures for the aforesaid problems, the number of parts in the optical deflecting device is increased, resulting in problems that assembly man-hour is increased, cost is increased and an optical deflecting device needs to be large in size.